In nuclear reactors having power and shutdown control rods, it is known to use guide tubes not only for guiding the rods themselves in the form of a bundle, but also for guiding the shaft connecting the bundle to the mechanism controlling the translation of the rods. This shaft, called an extension piece, is centered and guided in a tube arranged in the space located between the guide tubes of the bundle and the wall of the containment carrying the mechanisms.
Arranged in a conventional manner at the end part of the tube is a ring which ensures the guidance of the extension piece and acts as a flow limiter to force the coolant, which has passed through the core and penetrated into the guide tubes, to flow off through the orifices provided for this purpose in the guide tubes of the bundles.
Moreover, during the operations of unloading and reloading the core, this ring allows the extension piece to be guided laterally, so that, when the cover of the vessel is lowered, this extension piece can penetrate into perforations made in the cover.
Finally, it must be possible to remove this ring so that the conventional maintenance operations can be carried out on the components of the nuclear-reactor cores.
For this purpose, this ring is retained on the end plate of the tube by means of flexible elements. These flexible elements are integral with the end plate and are distributed circumferentially round the orifice made in the center of the said plate. The end part of the flexible elements which is opposite that connected to the plate has an extra thickness intended for bearing on the peripheral edge of the ring, when the latter is arranged in the orifice, thus ensuring that it is retained.
Consequently, a constant mechanical tension is generated in the flexible elements. Because of the atmosphere prevailing in the core of the reactor during its operation, this assists the development of stress corrosion and the risks of cracking of the flexible elements. These risks of cracking are all the more harmful because they can cause fractures of the flexible elements. The immediate results of these fractures are that the rings lose their retention and objects migrating in the primary circuit of the nuclear reactor are produced.
In an embodiment known from European Patent Application 0,133,173, the guide ring is automatically snapped into the orifice in the end plate by means of retractable attachment elements which are carried by the ring and which are held in the locking position by means of spring devices.
Although these removable systems are convenient to install and function very safely, it is still just as likely that, as a result of stress corrosion phenomena, there will be risks of fracture, causing migrating bodies to be produced.
Furthermore, these systems require relatively complex tools to fit them and remove them, because these operations are carried out under a considerable depth of water necessary to protect the operators.